(1) Field of the Invention
The present invention relates to low-viscosity pigment aqueous ink for a ball-point pen and to a ball-point pen. More specifically, the present invention is directed to an aqueous ink to be supplied to a so-called direct reservation type ball-point pen using an ink reservoir which directly reserves ink without using an ink absorber body or an ink feeder, and to a ball-point pen using the aqueous ink.
(2) Description of the Prior Art
A ball-point pen includes a writing tip consisting of a ball and a tip holder, an ink reservoir, a barrel, and the like components. Writing by a ball-point pen of this sort is done in such a manner that ink flowing out from the inside of the writing tip as the ball in the tip rotates or rolls transfers onto or penetrates into a recording medium such as paper. Traces of writing are particularly formed by the transfer.
An aqueous ink for a ball-point pen has a low viscosity of a few mPa.multidot.S or lower and therefore a ball-point pen using the aqueous ink has the advantage of allowing writing at low writing pressures. This results in a good touch of writing. However, aqueous ball-point pens readily bring about a forward flow phenomenon in which ink flows out from the writing tip or, conversely, a backward flow phenomenon in which the air enters from the tip of a ball-point pen to flow out ink. Conventionally, these phenomena are prevented by use of a fiber sliver called inner cotton. Also, if a ball-point pen of this sort is left unused with no cap on it, the vapor pressure of a solvent increases to dry up the solvent. Consequently, the writing tip dries and ink no longer flows out, making writing impossible.
On the other hand, conventionally known oil type ball-point pens use high-viscosity ink having a viscosity of a few thousand mPa.multidot.S or higher. This results in a bad touch of writing because the ball rotates with a high resistance when ink flows out from the writing tip. Also, only a small amount of ink flows out from the writing tip during writing. This leads to ink-blotting, unevenness of writing traces, a low writing density, and the need for a high writing pressure.
As an improvement of these oil type ball-point pens, a ball-point pen for aqueous ink, called medium-viscosity ink, having a medium viscosity (a few mPa-S to a few thousand mPa.multidot.S) between the viscosities of the aqueous ink and the oily ink described above, has recently been developed. This ball-point pen makes use of aqueous ink with a relatively low viscosity which has a so-called shear thinning viscosity by which the viscosity decreases due to rotation of the tip ball and ink smoothly flows out. Unfortunately, since ink readily dries in this ball-point pen, a cap for sealing the writing tip is necessary. Also, the low viscosity readily brings about an ink backward or forward flow phenomenon. Furthermore, unclear traces of writing easily take place due to falling or knocking impacts. Accordingly, no ideal low-viscosity aqueous ink ball-point pen has been obtained yet because a touch of writing, backward and forward flow phenomena, cap-off performance, and a pen structure influence each other.
Japanese Patent Application Laid-open No. Hei 6-57194 has disclosed ink for aqueous ball-point pens, which uses a dye, a moisture retaining and wetting agent, and a crosslinking acrylic acid polymer as a thixotropic property imparting agent. Pigments have no resolubility characteristics, and so their dispersions become unstable with time. Therefore, pigments are inferior to dyes in the performance as aqueous ink for use in direct reservation type ball-point pens. Furthermore, Japanese Patent Publication No. Hei 6-57194 describes that pigments wear the ball in the writing tip during writing and thereby shorten the life of writing.
Generally, when dyes are used the ink components neither easily dry nor solidify in the writing tip. This prevents drying of the writing tip while a cap is removed and allows a smooth outflow of ink during writing. Dyes are superior to pigments in a so-called non-drying property. However, when used in a cap-less, portable ball-point pen, dyes are unsatisfactory in the properties of preventing a flow of ink from the writing tip, a so-called forward flow, when the writing tip is directed downward. This may contaminate user's clothes or the like.
Ink using natural resins such as xanthan gum and guar gum, acryl- and urethane-based synthetic polymers, and inorganic substances such as smectite and montmorillonite is also known. Unfortunately, this type of ink has an unsatisfactory moisture retention in the writing tip. Therefore, if a ball-point pen using the ink is left unused with no cap on it, unclear traces of writing may occur due to dry-up and, in the worst case, writing becomes impossible. This results in the disadvantage that the ball-point pen must be capped after being used.
In ball-point pens for low-viscosity aqueous ink, a greasy semitransparent non-drying substance called a follower is normally filled in the rear end of an ink reservoir. To prevent ink evaporation, therefore, it is only necessary to reduce evaporation through a gap between the writing tip, i.e., the ball, and the holder. As an example, it is unnecessary to take account of evaporation of ink if a solvent having a very low vapor pressure is primarily used as in the case of oil type ball-point pens. However, water is a prime solvent in aqueous ink, and so it is not possible to prevent evaporation of water only by proper selection of a low-vapor-pressure solvent.
To solve the above problems, a ball-point pen using low-viscosity aqueous ink is possible. Unfortunately, the ink outflow amount is still large in this ball-point pen. To prolong the life of writing, therefore, it is necessary to increase the ink filling amount by increasing the diameter of an ink reservoir. Also, since the ink viscosity is low, a gap may be formed between the tip ball and the tip holder when the writing tip is pointed down. If this is the case, ink oozes out (to cause a forward flow) or the ink outflow amount increases in writing. Furthermore, after writing is done with the writing tip pointed up or when a knocking or falling impact is applied, ink immediately below the tip ball tends to flow backward, resulting in unclear traces of writing.